The present invention relates to a magnetoresistive memory device that utilizes a magnetoresistance effect. In particular, the present invention relates to a magnetoresistive memory device suitable for high-speed pulse transmission.
A magnetic random access memory (MRAM) has been studied as a solid-state memory device including magnetoresistive elements (MR elements). The MRAM is provided with a conductive wiring that includes word lines and sense lines: the word lines are used to generate a magnetic field that is applied to the MR elements for recording and the sense lines are used for reading. A conventional MRAM uses a NiFe film or the like that has an anisotropic magnetoresistance (AMR) effect, expressed by a magnetoresistance ratio (MR ratio) of about 2%. However, it is difficult to increase the output of this MRAM. Ever since an artificial lattice film, in which magnetic films are exchange-coupled via a non-magnetic film, was proved to exhibit a giant magnetoresistance (GMR) effect, a MRAM using a GMR film has been proposed. The GMR film, including antiferromagnetically exchange coupled magnetic films, has a large MR ratio. Compared with the AMR film, however, the GMR film requires a larger magnetic field, so that the current for recording/reading information is increased. In contrast to the exchange-coupling GMR film, a non-coupling GMR film is typified by a spin-valve film. The spin-valve film has various configurations, i.e., it may include an antiferromagnetic film or (semi) hard magnetic film. The spin-valve film can provide a larger MR ratio than the AMR film with a magnetic field as low as that required for the AMR film. In addition to the GMR film including, e.g., a Cu conductor film as a non-magnetic layer, a tunnel GMR (TMR) film including, e.g., an Al2O3 insulating film as a non-magnetic layer also has been used in the MRAM. The RAM that utilizes the magnetoresistance effect can form in principle a nonvolatile memory and is advantageous in achieving both high speed and high integration. Therefore, the RAM shows great promise as a next-generation memory.
The nonvolatile memory now in use is mainly a flash memory. The flash memory is voltage-driven, that is, the MOS transistor is driven with a high-speed voltage pulse during writing operation. A ferroelectric memory, which is still in the research and development stage, also is voltage-driven.
In contrast, the MRAM is a device that is current-driven. To record information on a MR element, a pulse magnetic field needs to be generated by applying a pulse current to conductive wires (word lines) that are arranged around the MR element. Therefore, when the pulse waveform is distorted, the operation of the MRAM becomes unstable. Thus, the mismatch of impedance of the word lines makes it difficult for the MRAM to operate at high speeds.
With an improvement in the degree of integration, the MRAM causes crosstalk easily. The crosstalk is magnetic field noise that is brought to the MR elements from a pulse current transmitted on the adjacent word lines. This noise interferes with an improvement in the degree of integration because it may erase the recorded information.
A word line of a single wire is used conventionally to write information into a MR element. However, there is a limit to impedance matching of the conventional word line, and crosstalk cannot be suppressed sufficiently. Therefore, the present invention employs word lines of a plurality of wires that extend in the same direction for a MR element. A magnetoresistive memory device of the present invention includes a magnetoresistive element and a wiring for applying a magnetic field to the magnetoresistive element. The wiring includes two or more conductive wires that extend in the same direction.
The present invention easily can provide the impedance matching of a pulse transmission line, thus decreasing a delay coefficient and suppressing the distortion of a pulse waveform. Therefore, a high-speed response can be achieved in a MRAM. Moreover, the present invention can make the coupling between adjacent word lines relatively weak, so that crosstalk in the MRAM can be reduced easily.